Topographical models of geographical areas may be used for many applications. For example, topographical models may be used in flight simulators and for planning military missions. Furthermore, topographical models of man-made structures (e.g., cities) may be extremely helpful in applications such as cellular antenna placement, urban planning, disaster preparedness and analysis, and mapping, for example.
Various types and methods for making topographical models are presently being used. One common topographical model is the digital elevation map (DEM). A DEM is a sampled matrix representation of a geographical area which may be generated in an automated fashion by a computer. In a DEM, coordinate points are made to correspond with a height value. DEMs are typically used for modeling terrain where the transitions between different elevations (e.g., valleys, mountains, etc.) are generally smooth from one to a next. That is, DEMs typically model terrain as a plurality of curved surfaces and any discontinuities therebetween are thus “smoothed” over. Thus, in a typical DEM no distinct objects are present on the terrain.
On the other hand, a digital surface model (DSM) is a raster-based description of the terrain that includes objects on the terrain, such as buildings and vegetation. One approach for generating DSMs is to render objects manually or semi-automatically, and then add texture by manually “painting” the objects or superimposing imagery thereon. While this approach may provide a very realistic looking DSM of the area of interest, this approach is very labor intensive and thus cost prohibitive for many applications.
Other approaches have been attempted to generate textured DEMs using more automated (i.e., less labor intensive) approaches. One such approach is outlined in an E-Newsletter article entitled “Contours” from Tecplot, Inc., Issue 21, September 2003 (available at tecplot.com/showcase/contours/issue—21/article04.htm). This article discusses using Tecplot software to apply texture mapping to DEM files. More particularly, the article discusses how Tecplot may be used to texture map a single nadir image to a DEM.
While this approach may provide fairly realistic texturing for DEMs of relatively undeveloped terrain, this approach may not provide desired results for geographical areas of interest, such as cities, which include a large number of manmade objects. This is because buildings, towers, etc., have many different sides, and the features on the sides of such objects may not be adequately captured by a single nadir image.
One particularly advantageous 3D site modeling product is RealSite® from the present Assignee Harris Corp. The RealSite® site modeling product may be used to register overlapping images of a geographical area of interest, and extract high resolution DEMs using stereo and nadir view techniques. The RealSite® site modeling product renders 3D models of buildings (i.e., manmade structures) based upon manual operator selection of object vertices, shapes, etc., and these models are overlayed at the appropriate positions on a DEM. Various types of polygonal shapes may then be overlayed on the modeled objects, and images with the best collection angle for each polygon are automatically selected as a texture map based upon an obscuration test.
While the RealSite® site modeling product provides extremely realistic 3D DSMs, this approach may not be appropriate for certain implementations because of the potentially high labor costs associated with manually defining numerous building vertices, shapes, etc. for large urban areas. That is, the high level of detail and crisp object delineation provided by the RealSite® site modeling product may be greater than is required in certain circumstances.